1. Field of the Invention
The subject matter of this invention relates generally to high resistance grounded systems and more particularly to high resistance grounding schemes for oil well electrical systems.
2. Description of the Prior Art
Through the years oil well pump installations have utilized an ungrounded high voltage power system. A system of this type allows for the connection of an instrumentation signal system located deep in the well between the neutral of the oil well pump motor and the electrically conducting casing of the well. This saves considerable wiring expense, which would otherwise be necessary to traverse the great distance between the down hole sensor system and the surface signal measurement system. This system has a disadvantage in that the common mode voltage of a three-phase electrical system is not controlled because it is ungrounded and can thus reach extreme levels. This is hazardous to personnel and possibly destructive of electrical insulation. In such a system there is no intentional connection to ground. It has been found, though, that occasionally these ungrounded systems have exhibited unexplained, sometimes very wide spread, insulation failures with catastrophic results. It has been found that the source of these unexplained insulation failures often turns out to be an arcing ground fault condition deep in the well case structure. Severe voltage escalation occurs in these situations and this is what causes the unexplained insulation failures. The solution to this is to unground the power system by connecting the neutral point, supposedly at zero voltage to ground, through a high resistance. High resistance grounding has proven to be the most reliable form of power system grounding. It limits the available fault current to only a few amperes under ground fault conditions and in the event a ground fault occurs, one can continue to operate the system without the need to close down the circuit. This is a desirable feature for continuous process facilities where failure would result in significant losses. The high resistance connection provides damping for the voltage escalation thereby preventing a transient overvoltage from building up and causing failure. At the same time the resistance limits the available fault current to a very low value. The traditional way in the past of applying the high resistance grounding was to connect the neural point of a Wye connected surface power supply system to ground through a resistance, the value of which is selected to allow less than 10 amperes maximum current to flow under the worst case condition. For a Delta system, the neutral point is derived through three grounding transformers or a zig-zag grounding transformer. In the oil field industry, electrically submersible pumps and motors have been traditionally run ungrounded for surface continuity considerations as well as the extremely high cost of pulling the pump and motor up when it has faulted. This provides a perfect application for high resistance grounding. The high resistances grounding will allow the electrically submersible pumps to operate for a longer period of time under ground fault conditions. However, a frequent requirement of the oil well industry is to know what the down hole temperatures, pressures, etc. are. Existing down signals to the surface via line-to-ground connections using the power conductors as part of the signal path. A small signal is superimposed on the power lines and is transmitted to the surface. In this manner extremely expensive control wires of up to 15,000 feet in length are not required. As long as the power system is ungrounded this method is cost effective. However, it does not provide the transient over voltage protection provided by a high resistance grounding system, as the high resistance grounding system will short out the signals on the power conductors. It would be advantageous therefore, if an electrical power system for a down-hole oil well pump could be found that had all the advantageous of high resistance grounding as described previously, but which would also allow for the utilization of the power wiring to carry sensor signals from deep in the well. In the recent past, a system has been found to accomplish both purposes. High resistance grounding is provided but left unconnected until it is needed, as would be the case if an arcing ground fault were detected. That means that signals from an electronic monitoring systems deep in the well can be carried on the power lines of the pump to the surface, utilizing the well casing as a ground conductor. However, if an arcing ground fault occurs, the presence of common mode voltage variation can be quickly sensed at the surface and the high resistance ground can then be quickly inserted into the circuit to limit current and voltage excursions. Once this happens the control system signals are swamped out, but that is an acceptable compromise. At this point in time the protection of the personnel and equipment becomes more important. In the past, this system has utilized a gas discharge switch or tube in series with the grounding resistance. The grounding resistance is interconnected, for example, between the neutral of the power supply transformer and the aforementioned gas discharge tube in turn is interconnected to ground. If the voltage of the neutral of the aforementioned power supply transformer is at zero, then no current flows through the gas discharge tube and it remains an open circuit. If an arcing ground fault begins to cause the common mode voltage at the neutral of the transformer to build up, the current through the high resistance grounding system and the serially connected gas discharge tube causes the gas discharge tube to flash over or conduct thus connecting the high resistance to ground, thus bringing the voltage on the neutral of the aforementioned power transformer back to ground potential. This prevents dangerous arcing ground faults and extreme levels of voltage excursion and also provides a current limiting function. This system has a disadvantage in that the break-over voltage and conduction characteristics of the gas discharge tube, once chosen are fixed for each value of gas discharge tube utilized. It would be advantageous if a high resistance grounding system could be utilized, which was controlled to be in the off state during a time period when it was not needed, but which would be controlled to be turned on by way of a highly reliable system when needed. In such a system, values of current, voltage, etc. could be programmed into the system to provide a wide range of application in a single system. It would be advantageous if such a system could be found which improved the safety of the overall system, which could be used on either Wye or Delta transformers and which provided continuous operation during all ground fault conditions and also provided an alarm to advise personnel of ground fault condition.